Fats and oils (the expression "fats and oils" as used herein means triglycerides) and fatty acid esters (the expression "fatty acid esters" as used herein means esters, except triglycerides, of fatty acids and lower or higher alcohols) generally contain at least several to several tens of ppm of sulfur. These raw materials will hereinafter sometimes be referred to as "starting materials" or "starting oils". When hydrogenated fatty acids, aliphatic alcohols or aliphatic amines are to be produced from these starting oils, a trace amount of sulfur compounds contained therein would deteriorate a hydrogenation catalyst employed in each production step and thus cause a marked reduction of its catalytic life. In particular, when these starting oils are catalytically reduced with hydrogen in the presence of a catalyst for ester reduction, a trace amount of sulfur compounds contained therein would act as a catalyst poison and thus seriously reduce the duration of the catalyst for ester reduction.
Under these circumstances, the present inventors have conducted studies of purification methods for the aforesaid starting oils to reduce the sulfur content thereof and, as a result, brought out the following problems.
(1) Problem of Purification by Distillation
When methyl esters of fatty acids which are derived from natural fats and oils in a conventional manner are subjected to distillation in a yield of 90% or 98%, the sulfur content can be reduced to 10% or 20%, respectively, of the initial content. However, where fatty acid methyl esters which are usually available or prepared are distilled for meeting the purpose of sulfur content reduction, there is an unavoidable distillation loss of at least 5%, and the alkyl distribution of the starting material varies largely.
In the case of fats and oils or fatty acid esters of higher alcohols, because of the high boiling point thereof, it is difficult to remove the sulfur compounds from such starting materials by distillation.
(2) Problems of Purification with Catalyst for Desulfurization
In the field of petroleum refining, molybdenum or tungsten catalysts are used for removing sulfur compounds from light oil and heavy oil as disclosed, for example, in Shokubai Process Kacaku (Catalytical Process Chemistry), published by Tokyo Kagaku Dojin Shuppan, pages 377-408 (1984).
The catalysts require temperatures of 300.degree. C. or higher for manifestation of the desulfurization activity. If fats and oils or fatty acid esters are hydrogenated in such high temperatures, hydrogenolysis of the ester group occurs, which brings about an increase in the acid value (AV) and a marked increase of decomposition products of the starting material. In addition, a catalyst component is dissolved by the produced fatty acids which adversely affects selectivity of the catalyst in the ester reduction reaction.